1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine, a facsimile machine, and a printer.
2. Description of the Related Art
There is known an image forming apparatus that holds a recording sheet by a transfer nip formed between an image carrier and a contact unit that are in contact with each other and that transfers a toner image being a visible image formed on the image carrier to the recording sheet. In the configuration, if a cardboard is used as the recording sheet, a moving speed of the surface of the image carrier may be momentarily changed by sharp load fluctuation when a leading edge of the cardboard is caused to enter the transfer nip or when a trailing edge thereof is caused to exit from the transfer nip. The change in the moving speed of the surface causes liner uneven density in the image.
Meanwhile, Japanese Patent Application Laid-open No. H4-242276 describes an image forming apparatus that adjusts a distance between an image carrier and a contact unit in the following manner. That is, the image forming apparatus detects the thickness of a recording sheet by a thickness sensor before the recording sheet enters a nip between the image carrier and the contact unit. A moving unit forcibly moves the contact unit away from the image carrier before the leading edge of the recording sheet is caused to enter the nip therebetween. With this feature, a gap of 30% to 90% of a sheet thickness detected by the thickness sensor is provided between the image carrier and the contact unit, and then the recording sheet is caused to enter therebetween. This configuration allows reduction in sharp load fluctuation on the image carrier upon entering of the leading edge of the recording sheet into the gap and upon discharging of the trailing edge thereof from the gap, as compared with a case in which the gap is not provided. This enables minimization of linear uneven density.
Japanese Patent Application Laid-open No. 2001-92332 describes an image forming apparatus that changes a distance between an image carrier and a contact unit in the following manner. That is, first, the contact unit is separated from the image carrier by a predetermined distance before a leading edge of a recording sheet is caused to enter a nip between the image carrier and the contact unit. This allows reduction in sharp load fluctuation on the image carrier upon entering of the leading edge of the recording sheet into the nip. Subsequently, the contact unit is brought close to the image carrier right after the leading edge of the recording sheet is caused to enter the nip between the contact unit and the image carrier mutually separated from each other, to obtain predetermined transfer pressure. Then, the contact unit is again moved away from the image carrier right before the trailing edge of the recording sheet is caused to exit from the nip. This allows reduction in load sharp fluctuation on the image carrier upon exit of the trailing edge of the recording sheet from the nip.
However, in the image forming apparatus described in Japanese Patent Application Laid-open No. H4-242276, a contact depth between the image carrier and the contact unit is not calculated at all, and thus, there is a high probability to cause transfer failure due to low transfer pressure. Specifically, a transfer nip is generally formed as a contact portion between the image carrier and the contact unit to keep a longer transfer time. The surface of a material of at least one of the image carrier and the contact unit is formed with an elastically deformable element, and the element is elastically deformed at the contact portion to form a wide nip. One of the image carrier and the contact unit is biased toward the other one by a spring while the other one is movably held. This is because the element biased by the spring is caused to follow the thickness of a recording sheet entering the transfer nip and escape the other side, and thus, even if the recording sheet is the cardboard, appropriate transfer pressure is obtained while it is caused to reliably enter the transfer nip.
In this configuration, it is assumed that either one of the image carrier and the contact element is forcibly moved by a moving unit from a state of not holding the recording sheet in the transfer nip and is separated little by little from the other one. Then, the size of the transfer nip becomes smaller and smaller, and eventually the element that is moved is separated from the other one. A moving distance required for the separation is nearly the same value as the contact depth between the image carrier and the contact element before the movement.
For example, as shown in FIG. 15, a contact unit 902 is structured so as to be elastically deformable and movable, and the bottom face of a bearing 908 that bears a rotating shaft of the contact unit 902 is biased by a spring 905, to thereby bring the contact unit 902 into contact with an image carrier 901. A contact depth of the image carrier 901 into the contact unit 902 is K1 at this time. By rotating an eccentric cam (not shown) from this state, pressing a cam face thereof against a top surface S of the bearing 908, and forcibly depressing the contact unit 902 downward in FIG. 15, a following state is obtained.
Specifically, as shown in FIG. 16, the contact unit 902 starts separating from the image carrier 901 at a point in time when the moving amount of the contact unit 902 starts exceeding the contact depth K1 caused by forcible depression. In this manner, to cause the contact unit 902 to start separating from the image carrier 901, at first, the contact unit 902 needs to be moved by a distance equal to the contact depth K1. In the image forming apparatus described in Japanese Patent Application Laid-open No. H4-242276, the contact unit is further moved thereafter, and the gap of 30% to 90% of the thickness of the cardboard is provided between the image carrier and the contact unit. The contact depth K1 is set according to a structure of a model; however, if it is an ordinary set value, the contact depth K1 becomes often a value considerably greater than a sheet thickness depending on the thickness of the cardboard. In this case, if the contact unit is separated from the image carrier, required transfer pressure cannot be obtained depending on the thickness of the cardboard, which may cause transfer failure.
In the image forming apparatus described in Japanese Patent Application Laid-open No. 2001-92332, the contact unit is brought close to the image carrier right after the leading edge of the recording sheet is caused to enter between the image carrier and the contact unit to ensure desired transfer pressure. Even so, the transfer failure may occur. Specifically, after the leading edge of the recording sheet is caused to enter therebetween, the contact unit is brought close to the image carrier. The desired transfer pressure cannot be obtained until this operation is completed, and this may cause transfer failure in a region of the leading edge of the recording sheet. Moreover, when the contact unit is moved away from the image carrier before the trailing edge of the recording sheet is caused to exit from the nip between the image carrier and the contact unit, the desired transfer pressure cannot also be obtained, and this may cause transfer failure also in a region of the trailing edge of the recording sheet. In recent years in which the carrying speed of a sheet is being increased to implement high-speed printing, even if a high-speed moving mechanism capable of moving the contact unit at high speed is provided, it is difficult to eliminate a region where transfer failure may occur in the leading edge and the trailing edge of the recording sheet.